1. Field of the Invention
This invention relates to a method of making abrasive compositions, and more particularly, it relates to a method of making abrasive compositions comprised of aqueous suspensions of water-insoluble abrasive polishing agents and humectant, with post-reactor dewatering and sizing of the abrasive particles being performed directly via hydraulic chamber press filtering combined with vacuum dewatering, followed by wet comminution, and this invention also relates-to products of this process scheme.
2. Description of the Related Art
An abrasive substance has been included in conventional dentifrice compositions in order to remove various deposits, including pellicle film, from the surface of teeth. Pellicle film is tightly adherent and often contains brown or yellow pigments, which impart an unsightly appearance to the teeth. While cleaning is important, the abrasive should not be so aggressive so as to damage the teeth. Ideally, an effective dentifrice abrasive material maximizes pellicle film removal while causing minimal abrasion and damage to the hard tooth tissues. Consequently, among other things, the performance of the dentifrice is highly sensitive to the abrasive polishing agent ingredient. Conventionally, the abrasive polishing material has been introduced in flowable dry powder form to dentifrice compositions, or via re-dispersions of flowable dry powder forms of the polishing agent prepared before or at the time of formulating the dentifrice.
A number of water insoluble, abrasive polishing agents have been used or described for dentifrice compositions. These abrasive polishing agents include natural and synthetic abrasive particulate materials. The generally known synthetic abrasive polishing agents include amorphous precipitated silicas, silica gels, dicalcium phosphate and its dihydrate forms, calcium pyrophosphate and precipitated calcium carbonate (PCC). Other abrasive polishing agents for dentifrices have included chalk, magnesium carbonate, zirconium silicate, potassium metaphosphate, magnesium orthophosphate, tricalcium phosphate, and the like.
Synthetically-produced precipitated silicas, in particular, have been used as abrasive components in dentifrice formulations due to their cleaning ability, relative safeness, and compatibility with typical dentifrice ingredients, such as humectants, thickening agents, flavoring agents, anti-caries agents, and so forth. As known, synthetic precipitated silicas generally are produced by the de-stabilization and precipitation of amorphous silica from soluble alkaline silicate by the addition of a mineral acid and/or acid gases under conditions in which primary particles initially formed tend to associate with each other to form a plurality of agglomerates (i.e., discrete clusters of primary particles), but without aggregation into a three-dimensional gel structure. The resulting precipitate is separated from the aqueous fraction of the reaction mixture by filtering, washing, and drying procedures, and then the dried product is mechanically comminuted in order to provide a suitable particle size.
The silica drying procedures are conventionally accomplished using spray drying, nozzle drying (e.g., tower or fountain), flash drying, rotary wheel drying, oven/fluid bed drying, and the like, which often require considerable expenditures for equipment and operating costs. A similar issue is associated with other synthetically derived polishing agents, such as silica gel and PCC.
Additionally, conventional abrasive polishing agents intended for dentifrices have required comminution in order to reduce the particle size of the dried precipitated silica product down to a size that does not feel gritty in the mouth of a dentifrice user, while, on the other hand, not being so small as to lack sufficient polishing action. That is, in conventional practice, the median particle size of the silica in the reactor formed by acidulation of a metal silicate is too large for dentifrice applications and the like. To comminute dry silica particulates, grinding and milling equipment have been used, such as a hammer or a pendulum mill used in one or multiple passes, and fine grinding has been performed, for example, by fluid energy or air-jet mill. These additional dry comminution operations entail added cost and process time.
U.S. Pat. No. 3,506,757 to Salzmann describes liquid dentifrices comprising particulate abrasive materials, stably suspended in an aqueous liquid vehicle with the aid of a polysaccharide gum as suspending agent. Similarly, PCT published application no. WO 97/46485 describes silica having a median particle size generally around 12 to 30 μm provided in the form of a suspension, which can be stabilized using a hydrocolloid, particularly naming polysaccharides such as xanthan gum, guar gum, and water-soluble cellulose ethers. U.S. Pat. No. 5,310,543 describes liquid dentifrices containing particulate siliceous abrasive cleaning agents stably suspended in a liquid medium with the aid of a polysaccharide gum and using a liquid medium specified as being substantially free from polyol-type humectants in order to obtain satisfactory rheological properties.
Among other things, rheologically stable liquid abrasive compositions containing appropriately sized abrasive particles would be desirable that could be prepared without the need for costly drying and dry milling/comminuting post-treatments.